An accurate assessment of the physiological significance of triiodothyronine requires precise determinations of the concentration of the hormone in a variety of physiologic and pathological settings. Gas chromatographic and radioimmunoassay (RIA) methods have been developed in our laboratory to measure triiodothyronine. Though both appear to be accurate, its inherent simplicity, sensitivity and precision render the RIA method more suited for routine studies. Studies of T3 an T4 levels in tissues and quantitation of T4 to T3 conversion should help establish physiological significance of T3.